1. Field of the Invention
The present invention is related generally to X-ray mask blanks and, more particularly, to an improved process of growing quality membranes for use as X-ray mask blanks at optimized growth rates.
2. The Prior Art
X-ray masks are required in X-ray lithography systems for making microcircuits, memories and other high-density electronic components. An X-ray mask is a thin-film X-ray transparent membrane that is in tension and which has been patterned with an X-ray absorber material. The X-ray mask must be flat and possess good dimensional stability, tensile strength and tensile stress, and exhibit low defect densities, both when it is patterned and thereafter, to yield quality high-density electronic components. The tension in the membrane must be sufficiently high to achieve the required flatness yet must be below the limit of its tensile strength to prevent its fracture.
The tension in the membrane now is obtained in one of two ways: a/ the membrane is formed in slack form and then mechanically stretched over a rigid frame to the desired tension; or b/ the membrane is deposited from vapor on a sacrificial substrate to a level of residual stress determined by deposition process parameters, and the substrate chemically etched away, leaving a taut membrane on a ring of the remaining substrate material. See "Advances in X-Ray Mask Technology," A. R. Shimkunas, Solid State Technology, September 1984, pp. 192-199.
The mechanical stretching technique requires the handling, stretching and bonding of thin films, frequently less than 5,000 nanometers in thickness. Such operations are delicate, time consuming and also often result in the loss of a significant portion of the membranes so produced.
In the second technique, the resultant residual stress in the membrane is a function of the selected deposition process parameters, including the reactant ratio, the temperature of the reactant gases forming the vapor during their reaction, the temperature of the substrate during deposition, and the post-deposition annealing cycle. Materials suitable as X-ray mask membranes, such as for example boron nitride, typically are formed in a radio-frequency driven plasma or by chemical vapor deposition, usually at low pressure and high temperature, and with substrate temperatures ranging anywhere from ambient to about 1,000.degree. C. The boron nitride thin films usually are generated by low pressure chemical vapor deposition, using diborane (B.sub.2 H.sub.6) and ammonia (NH.sub.3) in a background gas such as nitrogen (N.sub.2). See "The Chemical Deposition of Boron-Nitrogen Films," A. C. Adams et al, J. Electrochem. Soc., 127, (1980) pp. 399-405. The residual stress in the resultant thin boron nitride film depends primarily on the ratio of the diborane gas to the ammonia gas and upon the temperature of the substrate. For example, a high diborane gas to ammonia gas ratio coupled with a lower substrate temperature usually results in a higher residual tensile stress. On the other hand, a low diborane gas to ammonia gas ratio combined with a higher substrate temperature will introduce compressive stresses into the resultant thin film, rendering such films undesirable and unsuitable for use as X-ray mask blanks. The quality of the membranes, such as transparency, yield strength, defect density, crystallographic form and creep resistance, also are critically dependent on the deposition process parameters. These deposition process parameters, for the most part, must be adjusted so as to yield a compromise between film quality, membrane residual stress, and film growth rate.